The ash cloud began to reach London’s Heathrow airport — the world’s busiest international air travel hub — around lunchtime, a computer model indicated. The European air traffic control organization Eurocontrol reported about 500 flights in British airspace would be canceled Tuesday, roughly double the number expected earlier in the day.

The ash cloud was projected to cover all of British airspace by early Wednesday morning and will be densest over Scotland, Northern Ireland and northern England, according to Britain’s weather agency, the Met Office.

Glasgow airport in Scotland closed at 7 a.m. (2 a.m. ET), and both main airports in Belfast, Northern Ireland, were due to close at 1 p.m. (8 a.m. ET), Britain’s Civil Aviation Authority said. The restrictions were due to last until 7 p.m. (2 p.m. ET) and included nine other airports in Scotland and Northern Ireland, the CAA said.

In Ireland, the airports in Dublin and three other cities were closed to flights “until further notice,” the Irish Aviation Authority said. The IAA lifted earlier restrictions on other airports, including Shannon, but said they could be reimposed later.

Philippine authorities have said a large-scale eruption of the 2,464-meter (8,077-foot) peak is imminent, and have begun trying to evacuate about 50,000 people living around the nation’s most active volcano.

Gwendolyn Pang, the secretary-general of the Philippine Red Cross, said the ground around the mountain shook several times Wednesday. Emergency workers have so far evacuated 30,751 people, with 21 centers set up to take in the evacuees, she said.

People in surrounding Albay province have flocked to town centers to catch a glimpse of glowing lava cascading down the slopes of Mayon since the mountain began oozing fiery lava and belching clouds of ash this week.

The Philippine Institute of Volcanology and Seismology raised its alert level for the Mayon volcano Monday night, warning that a full-scale eruption could occur “within weeks to days.” The volcano, about 500 kilometers (310 miles) south of the Philippine capital Manila, has gone off 49 times since the first documented eruption in 1616.